Navigational reference dislodgement detection method and system

ABSTRACT

A method of tracking a position of a catheter within a patient includes securing a navigational reference at a reference location within the patient, defining the reference location as the origin of a coordinate system, determining a location of an electrode moving within the patient relative to that coordinate system, monitoring for a dislodgement of the navigational reference from the initial reference location, for example by measuring the navigational reference relative to a far field reference outside the patient&#39;s body, and generating a signal indicating that the navigational reference has dislodged from the reference location. Upon dislodgement, a user may be provided with guidance to help reposition and secure the navigational reference to the initial reference location, or the navigational reference may be automatically repositioned and secured to the initial reference location. Alternatively, a reference adjustment may be calculated to compensate for the changed reference point/origin.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/647,277, filed 29Dec. 2006, now pending, which is hereby incorporatedby reference as though fully set forth herein.

BACKGROUND OF THE INVENTION

a. Field of the Invention

The instant invention relates generally to the navigation of a medicaldevice through a patient. More specifically, the instant inventionrelates to a method and system for detecting and controlling for themovement of a reference point utilized in a localization system employedin navigating a medical device through a patient, and in particularthrough the heart and vasculature of the patient.

b. Background Art

It is well known to generate heart chamber geometry in preparation forcardiac diagnostic or therapeutic procedures. Often, a mapping cathetertip is placed against the wall of the heart chamber and thethree-dimensional coordinates of the mapping catheter tip measured usinga localization system. The three-dimensional coordinates become ageometry point. Multiple measurements are taken as the mapping catheteris moved within the heart chamber, resulting in a cloud of geometrypoints (also referred to as “location data points”) that defines thegeometry of the heart chamber. Various surface construction algorithmsmay then be applied to wrap a surface around the cloud of geometrypoints to obtain a representation of the heart chamber geometry.

It is desirable for the three-dimensional coordinate system to have astable reference point or origin. While any stable position willsuffice, it is desirable for many reasons to utilize a reference pointthat is proximate to the mapping catheter. Thus, a catheter-mountedreference electrode is often inserted into the heart and positioned in afixed location, for example the coronary sinus, to establish the originof the coordinate system relative to which the location of the mappingcatheter will be measured.

It is known, however, that the stationary reference electrode may becomedislodged. For example, the mapping catheter may collide or becomeentangled with the reference electrode, or the physician moving themapping catheter may inadvertently jostle the catheter carrying thereference electrode. The reference electrode may also be dislodged bypatient movement.

When the reference electrode becomes dislodged, it effectively shiftsthe origin of the coordinate system relative to which the position ofthe mapping catheter is measured. Unless the dislodgement is detectedand accounted for, positions of the mapping catheter measured after thedislodgement will be invalid.

BRIEF SUMMARY OF THE INVENTION

It is therefore desirable to be able to detect dislodgement of theelectrode or other navigational reference that defines the origin of thecoordinate system relative to which body geometries are measured.

It is also desirable to guide a user in reestablishing the originalposition of the navigational reference.

It is further desirable to provide a method by which the originalposition of the navigational reference may be reestablished.

In addition, it is desirable to provide a method that accounts for thedislodged location of the navigational reference without requiringreestablishment of the original position of the navigational reference.

Disclosed herein is a method of tracking a position of a catheter withina patient's body. The method includes the steps of: securing anavigational reference at an initial reference location within apatient's body; defining the initial reference location as a referencepoint for a coordinate system for locating positions of points in space;providing a moving electrode within the patient's body; determining alocation of the moving electrode and providing location information datafor the moving electrode, the location information data comprisingposition information that defines the location of the moving electrodein the coordinate system that uses the initial reference location as itsreference point; monitoring for a dislodgement of the navigationalreference from the initial reference location; and generating a signalindicating that the navigational reference has dislodged from theinitial reference location.

A user may be provided with guidance to help reposition and secure thenavigational reference to the initial reference location. Alternatively,a computer may be used to control a servo-controlled catheter toreposition and secure the navigational reference to the initialreference location. In still other embodiments of the invention, alocation of the navigational reference is determined after it hasdislodged from the reference location, and a reference adjustment iscalculated to compensate for the navigational reference having changedpositions. The reference adjustment may then be used to generatelocation information data for the moving electrode with reference to thecoordinate system that uses the initial reference location as itsreference point.

Dislodgement may be detected by monitoring an output signal from thenavigational reference for a value in excess of a threshold. Forexample, the velocity of the navigational reference may be monitored fora velocity in excess of a preset dislodgement threshold. In someembodiments of the invention, a far field reference is provided outsidethe patient's body, and both the navigational reference and the farfield reference are coupled to a circuit to generate an output signalthat is monitored for a signal indicative of dislodgement, for example asignal with an absolute amplitude above a dislodgment threshold. Theoutput signal is preferably filtered with both a high pass and a lowpass filter, and will typically be a signal indicative of the locationof the navigational reference relative to the far field reference. Thelow pass filter preferably has a cutoff frequency of 0.1 Hz, while thehigh pass filter preferably has a cutoff frequency of about 0.001 Hz. Itis understood that the filtering may be accomplished by filtering analogsignal, or the filtering process may also be accomplished using digitalsignal processing algorithms that operate on a digital signal, which maybe generated using analog-to-digital converters as is well known in theart.

Optionally, the moving electrode and the navigational reference includerespective first and second measurement electrodes to measureelectrophysiology information, and the output of the first measurementelectrode may be adjusted by compensating for at least one signal thatis common to each of the first and second measurement electrodes whenelectrophysiology measurements are taken simultaneously with the firstand second measurement electrodes.

Also disclosed is a method of measuring electrophysiology informationusing multiple electrodes that includes the steps of: providing alocalization system that determines locations of objects within athree-dimensional space and generates location information datacomprising position information determined relative to at least onereference; securing a local reference at an internal reference locationwithin a patient's body, the local reference comprising a firstmeasurement electrode; providing a far field reference outside thepatient's body at an external reference location; providing a secondmeasurement electrode within the patient's body; using the localizationsystem to determine a location of the second measurement electrode andproviding location information data for the second measurement electrodecomprising position information determined relative to the internalreference location as a reference point; simultaneously takingelectrophysiology measurements using each of the first and secondmeasurement electrodes and adjusting the output of the secondmeasurement electrode by compensating for at least one signal that iscommon to each of the first and second measurement electrodes;monitoring for a dislodgement of the local reference from the internalreference location; and generating a signal indicating that the localreference has dislodged from the internal reference location.Optionally, the method also includes: determining a dislodged locationof the local reference after it has dislodged from the internalreference location; and calculating an adjustment to compensate for achange in locations between the internal reference location and thedislodged location. Position information for the second measurementelectrode may then be determined relative to the internal referencelocation as a reference point by applying the adjustment to a locationof the second measurement electrode relative to the local reference inthe dislodged location.

According to another embodiment of the invention, a system for measuringelectrophysiology information using multiple electrodes includes: alocalization system that determines locations of objects within athree-dimensional space and that generates location information datacomprising position information determined relative to at least onereference; a local reference that can be secured at an internalreference location within a patient's body, the local referencecomprising a first measurement electrode that generates a firstmeasurement signal; a far field reference that can be secured at anexternal reference location outside the patient's body; a secondmeasurement electrode that can be placed within the patient's body, thesecond measurement electrode generating a second measurement signal; acommon mode processor to take electrophysiology measurements using eachof the first and second measurement electrodes and to adjust the outputof the second measurement electrode by removing at least one signalcomponent that is common to each of the first and second measurementsignals; an output processor coupled to the localization system thatdetermines a location of the second measurement electrode and provideslocation information data for the second measurement electrode, thelocation information data comprising position information determinedrelative to the internal reference location as a reference point; and acontroller that monitors for dislodgement of the local reference fromthe internal reference location and that generates a signal indicatingthat the local reference has dislodged from the internal referencelocation. Optionally, the system may further include one or more of thefollowing: a servo mechanism to reposition and secure the localreference to the internal reference location; an adjustment processor todetermine a dislodged location of the local reference and to calculate areference adjustment that compensates for the local reference havingmoved from the internal reference location to the dislodged location;and a filtering processor, including a high pass filter and a low passfilter, that outputs a filtered signal, which the controller monitorsfor an indication of dislodgement of the local reference from theinternal reference location.

A technical advantage of the present invention is that it may be used toalert a user to dislodgement of the navigational reference.

Another advantage of the present invention is that it may guide the userin correcting the dislodgement.

Yet another advantage of the present invention is that it canautomatically correct for the dislodgment, for example by automaticallyreestablishing the navigational reference in its original position, or,alternatively, by calculating a reference adjustment to compensate forthe changed position of the navigational reference.

The foregoing and other aspects, features, details, utilities, andadvantages of the present invention will be apparent from reading thefollowing description and claims, and from reviewing the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of a localization system utilized in anelectrophysiology study.

FIG. 2 is a flowchart that illustrates navigational referencedislodgement detection and mitigation functions according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention, which is preferably practiced in connection witha localization system, automatically detects dislodgement of anavigational reference for the localization system. In addition, thepresent invention provides methods of automatically correcting for thedislodgment or to guide a user, e.g., a physician, in repositioning thenavigational reference at its original location. For illustrativepurposes, the present invention will be described in the context of acardiac diagnostic or therapeutic procedure, such as anelectrophysiology study. One of ordinary skill in the art willappreciate, however, that the invention may be practiced with equalsuccess in any number of other applications, and, accordingly, theillustrative embodiment used herein to describe the invention should notbe regarded as limiting.

FIG. 1 shows a schematic diagram of a localization system 8 forconducting cardiac electrophysiology studies by navigating a cardiaccatheter and measuring electrical activity occurring in a heart 10 of apatient 11 and three-dimensionally mapping the electrical activityand/or information related to or representative of the electricalactivity so measured. System 8 can be used, for example, to create ananatomical model of the patient's heart 10 using one or more electrodes.System 8 can also be used to measure electrophysiology data at aplurality of points along a cardiac surface, and store the measured datain association with location information for each measurement point atwhich the electrophysiology data was measured, for example to create adiagnostic data map of the patient's heart 10. As one of ordinary skillin the art will recognize, and as will be further described below,localization system 8 determines the location of objects, typicallywithin a three-dimensional space, and expresses those locations asposition information determined relative to at least one reference.

For simplicity of illustration, the patient 11 is depicted schematicallyas an oval. Three sets of surface electrodes (e.g., patch electrodes)are shown applied to a surface of the patient 11, defining threegenerally orthogonal axes, referred to herein as an x-axis, a y-axis,and a z-axis. The x-axis surface electrodes 12, 14 are applied to thepatient along a first axis, such as on the lateral sides of the thoraxregion of the patient (e.g., applied to the patient's skin underneatheach arm) and may be referred to as the Left and Right electrodes. They-axis electrodes 18, 19 are applied to the patient along a second axisgenerally orthogonal to the x-axis, such as along the inner thigh andneck regions of the patient, and may be referred to as the Left Leg andNeck electrodes. The z-axis electrodes 16, 22 are applied along a thirdaxis generally orthogonal to both the x-axis and the y-axis, such asalong the sternum and spine of the patient in the thorax region, and maybe referred to as the Chest and Back electrodes. The heart 10 liesbetween these pairs of surface electrodes 12/14, 18/19, and 16/22.

An additional surface reference electrode (e.g., a “belly patch”) 21provides a reference and/or ground electrode for the system 8. The bellypatch electrode 21 may be an alternative to a fixed intra-cardiacelectrode 31, described in further detail below. It should also beappreciated that, in addition, the patient 11 may have most or all ofthe conventional electrocardiogram (ECG) system leads in place. This ECGinformation is available to the system 8, although not illustrated inFIG. 1.

A representative catheter 13 having at least one electrode 17 (e.g., adistal electrode) is also shown. This representative catheter electrode17 is referred to as the “roving electrode,” “moving electrode,” or“measurement electrode” throughout the specification. Typically,multiple electrodes on catheter 13, or on multiple such catheters, willbe used. In one embodiment, for example, localization system 8 maycomprise up to sixty-four electrodes on up to twelve catheters disposedwithin the heart and/or vasculature of the patient. Of course, thisembodiment is merely exemplary, and any number of electrodes andcatheters may be used within the scope of the present invention.

An optional fixed reference electrode 31 (e.g., attached to a wall ofthe heart 10) is shown on a second catheter 29. For calibrationpurposes, this electrode 31 may be stationary (e.g., attached to or nearthe wall of the heart) or disposed in a fixed spatial relationship withthe roving electrode 17, and thus may be referred to as a “navigationalreference” or “local reference.” The fixed reference electrode 31 may beused in addition or alternatively to the surface reference electrode 21described above. In many instances, a coronary sinus electrode or otherfixed electrode in the heart 10 can be used as a reference for measuringvoltages and displacements; that is, as described below, fixed referenceelectrode 31 may define the origin of a coordinate system.

Each surface electrode is coupled to the multiplex switch 24, and thepairs of surface electrodes are selected by software running on acomputer 20, which couples the electrodes to a signal generator 25. Thecomputer 20, for example, may comprise a conventional general-purposecomputer, a special-purpose computer, a distributed computer, or anyother type of computer. The computer 20 may comprise one or moreprocessors, such as a single central processing unit (CPU), or aplurality of processing units, commonly referred to as a parallelprocessing environment, which may execute instructions to practice thevarious aspects of the present invention described herein.

Generally, three nominally orthogonal electric fields are generated by aseries of driven and sensed electric dipoles (e.g., surface electrodepairs 12/14, 18/19, and 16/22) in order to realize catheter navigationin a biological conductor. Alternately, these orthogonal fields can bedecomposed and any pairs of surface electrodes can be driven as dipolesto provide effective electrode triangulation. Additionally, suchnon-orthogonal methodologies add to the flexibility of the system. Forany desired axis, the potentials measured across an intra-cardiacelectrode 17 resulting from a predetermined set of drive (source-sink)configurations are combined algebraically to yield the same effectivepotential as would be obtained by simply driving a uniform current alongthe orthogonal axes.

Thus, any two of the surface electrodes 12, 14, 16, 18, 19, 22 may beselected as a dipole source and drain with respect to a groundreference, such as belly patch 21, while the unexcited electrodesmeasure voltage with respect to the ground reference. The measurementelectrode 17 placed in the heart 10 is exposed to the field from acurrent pulse and is measured with respect to ground, such as bellypatch 21. In practice the catheters within the heart may containmultiple electrodes and each electrode potential may be measured. Aspreviously noted, at least one electrode may be fixed to the interiorsurface of the heart to form a fixed reference electrode 31, which isalso measured with respect to ground, such as belly patch 21. Data setsfrom each of the surface electrodes, the internal electrodes, and thevirtual electrodes may all be used to determine the location of themeasurement electrode 17 or other electrodes within the heart 10.

The measured voltages may be used to determine the location inthree-dimensional space of the electrodes inside the heart, such as theroving electrode 17, relative to a reference location, such as referenceelectrode 31. That is, the voltages measured at reference electrode 31may be used to define the origin of a coordinate system, while thevoltages measured at roving electrode 17 may be used to express thelocation of roving electrode 17 relative to the origin. Preferably, thecoordinate system is a three-dimensional (x, y, z) Cartesian coordinatesystem, though the use of other coordinate systems, such as polar,spherical, and cylindrical coordinate systems, is within the scope ofthe invention.

As should be clear from the foregoing discussion, the data used todetermine the location of the electrode(s) within the heart is measuredwhile the surface electrode pairs impress an electric field on theheart. The electrode data may also be used to create a respirationcompensation value used to improve the raw location data for theelectrode locations as described in U.S. Patent Application PublicationNo. 2004/0254437, which is hereby incorporated herein by reference inits entirety. The electrode data may also be used to compensate forchanges in the impedance of the body of the patient as described inco-pending U.S. patent application Ser. No. 11/227,580, filed on 15 Sep.2005, which is also incorporated herein by reference in its entirety.

In summary, the system 8 first selects a set of surface electrodes andthen drives them with current pulses. While the current pulses are beingdelivered, electrical activity, such as the voltages measured at leastone of the remaining surface electrodes and in vivo electrodes, ismeasured and stored. Compensation for artifacts, such as respirationand/or impedance shifting, may be performed as indicated above.

In a preferred embodiment, the localization/mapping system is the EnSiteNavX™ navigation and visualization system of St. Jude Medical, AtrialFibrillation Division, Inc. Other localization systems, however, may beused in connection with the present invention, including for example,the CARTO navigation and location system of Biosense Webster, Inc. Thelocalization and mapping systems described in the following patents (allof which are hereby incorporated by reference in their entireties) canalso be used with the present invention: U.S. Pat. Nos. 6,990,370;6,978,168; 6,947,785; 6,939,309; 6,728,562; 6,640,119; 5,983,126; and5,697,377

Various aspects and features of the present invention will now bedescribed with reference to FIG. 2. In use, a navigational reference orlocal reference, such as reference electrode 31, is secured at aninitial reference location, preferably internal to the body of patient11 (block 100). The initial reference location is defined as a referencepoint (e.g., the origin) of a coordinate system that may be used tolocate points in space. As it moves within heart 10, the location ofroving electrode 17 (block 110) may be measured relative to thecoordinate system having the initial reference location as its origin(block 120), thereby outputting position information that defines thelocation of roving electrode 17 in the coordinate system that uses theinitial reference location as its reference point.

As one of ordinary skill in the art will recognize, and as describedabove, reference electrode 31 may become dislodged from the initialreference location during the course of an electrophysiology study, forexample if the physician inadvertently tugs on the catheter 29 carryingreference electrode 31, effectively moving the origin of the coordinatesystem relative to which the position of roving electrode 17 is measuredand invalidating any positions of roving electrode 17 measured after thedislodgement. It is desirable, therefore, for a controller to monitorfor dislodgement of the navigational reference from the initialreference location, and, if such dislodgement occurs, to generate asignal indicative of the dislodgement, for example to alert the userthat a dislodgement has occurred.

Thus, the navigational reference may be coupled to a circuit in order togenerate an output signal, which may be monitored for an increase abovea threshold indicative of the navigational reference having dislodgedfrom the initial reference location. For example, the velocity of thenavigational reference may be monitored, and, if the velocity of thenavigational reference exceeds a preset threshold, one may conclude thatthe navigational reference has dislodged from the initial referencelocation; if the navigational reference remains stationary, its velocityis nominally zero. Similarly, an acceleration vector could be monitored,and again, any change that is above a threshold (near zero) would beindicative of movement. It should be understood that the threshold mayalso be implemented as a minimum acceptable value rather than a maximumacceptable value, such that dislodgment may be detected unless themonitored output signal is greater than the minimum acceptable valuethreshold.

In other embodiments of the invention, a far field reference, such asbelly patch 21, is provided outside the body of patient 11 (block 130).Of course, the far field reference may be spaced apart from the body.Dislodgment may be detected by monitoring a location of the navigationalreference relative to the far field reference (block 140). If thepositional relationship between the navigational reference and the farfield reference changes beyond a preset threshold, one may conclude thatthe navigational reference has become dislodged from the initialreference location. This may be accomplished, for example, by couplingboth the navigational reference and the far field reference to a circuitin order to generate an output signal, which may be monitored for asignal that is indicative of the navigational reference having dislodgedfrom the initial reference location.

Preferably, the output signal generated above is a displacement vectorof the navigational reference relative to the far field reference.Typically, where the navigational reference has not become dislodgedfrom the initial reference location, the displacement vector will benominally 0 (that is, approximately [0, 0, 0]). However, as one ofordinary skill in the art will recognize, there are error sourcesinherent in localization system 8, especially across distances thattypically exist between the navigational reference and the far fieldreference, and it is desirable to account for these error sources. Onesuch error source is DC or very low frequency drift. Accordingly, theoutput signal may be filtered with a high pass filter (block 150) havinga cutoff frequency of about 0.01 Hz, and more preferably about 0.001 Hz.Additional error sources are respiration, patient movement, and cardiacmotion (e.g., the beating of the heart 10), which tend to producevariations having frequency components of a fraction of 1 Hz in the caseof respiration, to greater than 1 Hz with harmonics ranging to severalHz for cardiac motion. Accordingly, it may be desirable to also filterthe output signal with a low pass filter (block 160) having a cutofffrequency of about 0.1 Hz to about 0.5 Hz, and more preferable about0.15 Hz, and it may be desirable to include a respiration compensationvalue as described above (block 170). The filtered output signal (block180) may be monitored for a signal that is indicative of thenavigational reference having dislodged from the reference location. Asdescribed above, it is desirable that the filtered output signal be adisplacement vector that is approximately 0; thus, the filtered outputsignal may be monitored for a signal with an absolute amplitude that isabove a dislodgement threshold (block 190). If such a signal isdetected, the user (e.g., a physician or clinician) may be warned (block200), for example with an audible alarm, a visual cue, or a both anaudible alarm and a visual cue.

In addition to providing automatic dislodgement detection, the presentinvention also provides dislodgement mitigation or correction.Typically, dislodgement mitigation involves the user (e.g., a physician)repositioning the navigational reference to the initial referencelocation, thereby re-establishing the original coordinate system. Insome embodiments of the invention, it is indeed contemplated that theuser may be provided with guidance, such as positional feedback of thenavigational reference or a graphic depiction of the dislodged locationof the navigational reference relative to the initial reference location(block 240), to assist the user in repositioning and securing thenavigational reference to the initial reference location. It is alsocontemplated that, upon detecting a dislodgement, a computer may beutilized to control a servo mechanism, such as that disclosed in U.S.application Ser. No. 11/647,300, filed 29 Dec. 2006 and entitled“Robotic Surgical System”, which is hereby incorporated by reference asthough fully set forth herein, to reposition and secure the navigationalreference to the initial reference location.

Occasionally, however, the navigational reference will reestablishitself in a new, stable position after dislodgement from the initialreference location. If this occurs, it is not necessary to repositionand secure the navigational reference to the initial reference location.Rather, the new, dislodged (or post-dislodgement) location of thenavigational reference may be used to define the origin of a newcoordinate system. Thus, after determining the post-dislodgementlocation of the navigational reference, a reference adjustment (e.g., acoordinate transformation) may be calculated that relates the originalcoordinate system (that is, the coordinate system having an origin atthe initial reference location) to the new coordinate system (that is,the coordinate system having an origin at the post-dislodgementlocation), thereby compensating for the movement of the navigationalreference from the initial reference location to the dislodged location(block 210). The location of roving electrode 17 may be measuredrelative to the new coordinate system, and, by applying the referenceadjustment (block 220), the location of roving electrode 17 may bedefined relative to the original coordinate system such that alllocation information data for roving electrode 17 may be expressedrelative to the original coordinate system having the initial referencelocation as its reference point (block 230).

It is contemplated that roving electrode 17 may also be utilized tomeasure electrophysiology information on the surface of heart 10,including, without limitation, voltages, impedances, and complexfractionated electrogram (CFE) information, such as discussed in U.S.application Ser. No. 11/647,276, filed 29 Dec. 2006 and entitled “Systemand Method for Mapping Electrophysiology Information Onto ComplexGeometry”, which is hereby incorporated by reference as though fully setforth herein. Electrophysiology information may also be measured by thenavigational reference, such as reference electrode 31. Since referenceelectrode 31 and roving electrode 17 are proximate each other, they willexperience common noise signals, such as those generated by patientmotion, respiration, and cardiac motion. Advantageously, bysimultaneously measuring electrophysiology information at both rovingelectrode 17 and reference electrode 31, the output of roving electrode17 may be adjusted by compensating for at least one signal that iscommon to both roving electrode 17 and reference electrode 31, forexample by subtracting the output from reference electrode 31 from theoutput from roving electrode 17 according to the principle of commonmode rejection.

Although several embodiments of this invention have been described abovewith a certain degree of particularity, those skilled in the art couldmake numerous alterations to the disclosed embodiments without departingfrom the spirit or scope of this invention. For example, thoughelectrode 31 has been described as the initial reference location, oneof ordinary skill in the art will recognize that all suitablenavigational references, including, without limitation, magnetic coilsensors, may be utilized instead of or in addition to electrode 31without departing from the spirit and scope of the present invention.Further, though belly patch 21 has been described as the far fieldreference, one of ordinary skill in the art will recognize that anysuitable reference located outside the patient's body at an externalreference location may be utilized as the far field reference.

All directional references (e.g., upper, lower, upward, downward, left,right, leftward, rightward, top, bottom, above, below, vertical,horizontal, clockwise, and counterclockwise) are only used foridentification purposes to aid the reader's understanding of the presentinvention, and do not create limitations, particularly as to theposition, orientation, or use of the invention. Joinder references(e.g., attached, coupled, connected, and the like) are to be construedbroadly and may include intermediate members between a connection ofelements and relative movement between elements. As such, joinderreferences do not necessarily infer that two elements are directlyconnected and in fixed relation to each other.

It is intended that all matter contained in the above description orshown in the accompanying drawings shall be interpreted as illustrativeonly and not limiting. Changes in detail or structure may be madewithout departing from the spirit of the invention as defined in theappended claims.

What is claimed is:
 1. A method of tracking a position of a catheterwith a localization system, comprising: defining an initial referencelocation of a navigational reference as a reference point for acoordinate system for locating positions of points in space; monitoringa velocity of the navigational reference for an indication of adislodgement of the navigational reference from the initial referencelocation; and generating a signal indicating that the navigationalreference has dislodged from the initial reference location when thevelocity of the navigational reference exceeds a preset dislodgementthreshold.
 2. The method according to claim 1, further comprisingproviding guidance to a user to help re-position and secure thenavigational reference to the initial reference location.
 3. The methodaccording to claim 1, further comprising: determining apost-dislodgement location of the navigational reference after it hasdislodged from the initial reference location; calculating a referenceadjustment to compensate for the navigational reference having changedpositions from the initial reference location to the post-dislodgementlocation of the navigational reference.
 4. The method according to claim3, further comprising using the reference adjustment to generatelocation information data for a moving electrode, the locationinformation data comprising position information that defines thelocation of the moving electrode with reference to a coordinate systemthat uses the initial reference location as its reference point.
 5. Themethod according to claim 1, further comprising: providing a far fieldreference; wherein the monitoring step comprises monitoring a rate ofchange in a location of the navigational reference relative to the farfield reference and generating a signal indicating dislodgement when therate of change in the location of the navigational reference relative tothe far field reference exceeds a preset threshold.
 6. The methodaccording to claim 1, further comprising: simultaneously measuring afirst electrophysiological signal using a moving electrode and a secondelectrophysiological signal using the navigational reference; adjustingan output of the first electrophysiological signal by compensating forat least one signal component that is common to each of the firstelectrophysiological signal and the second electrophysiological signal.7. The method according to claim 1, wherein the monitoring stepcomprises: coupling the navigational reference to a circuit to generatean output signal; and monitoring the output signal for an increase abovea threshold indicative of the navigational reference having dislodgedfrom the initial reference location.